The present invention relates to various forms of filled polymer electrical insulators which are resistant to tracking caused by high voltage.
Electrical insulators are commonly formed of porcelain. This material is subject to a number of disadvantages. It is (a) energy intensive, (b) unsuitable for encapsulating, or attaching metal hardware, (c) brittle, (d) subject to fracture in dangerous sharp flying pieces, (e) difficult to form in close tolerances, and (f) requires expensive processing equipment, i.e., high firing temperature kilns.
In order to overcome the above disadvantages, the prior art also discloses forming electrical insulators from a variety of polymers including polyesters or epoxy resins incorporating fillers, hydrated alumina particles, for the purpose of providing resistance of the structure to carbon tracking due to arcing at high voltage. A maximum suggested amount of this type of filler is on the order of 70%. U.S. Pat. No. 4,001,128 suggests that such water of hydration of the alumina evaporates to produce a porosity which is stated to be reduced by the inclusion of a minor portion of silica treated with silane. Such patent discloses the use of extremely fine grades of alumina and silica.
A major portion of the cost of insulators of the filled polymer type is the resin binder. However, apparently it was believed that the amount of inert filler could not be increased to a significantly higher level without degrading the product. This may be the reason why such filled polymers have not received wide acceptance as electrical insulators.